Lamb Dip of a Quadrupole Transition in H_{2}

The saturated absorption spectrum of the hyperfineless S(0) quadrupole line in the (2-0) band of H_{2} is measured at λ=1189  nm, using the NICE-OHMS technique under cryogenic conditions (72 K). It is the first time that a Lamb dip of a molecular quadrupole transition has been recorded. At low (150-200 W) saturation powers a single narrow Lamb dip is observed, ruling out an underlying recoil doublet of 140 kHz. Studies of Doppler-detuned resonances show that the redshifted recoil component can be made visible for low pressures and powers, and prove that the narrow Lamb dip must be interpreted as the blue recoil component. A transition frequency of 252 016 361 164 (8) kHz is extracted, which is off by -2.6 (1.6) MHz from molecular quantum electrodynamical calculations therewith providing a challenge to theory.